This invention relates to a magnetic recording medium in which magnetic particles are finely dispersed. More particularly, the invention is concerned with a magnetic recording medium having excellent magnetic properties and electromagnetic conversion characteristics owing to improved dispersibility of magnetic particles and, moreover, exhibiting good shelf stability and running performance in hot and humid environments.
Vinyl chloride-vinyl acetate-vinyl alcohol copolymers have been wide used as appropriate resins for magnetic tape binders that permit satisfactory dispersion of magnetic particles. The copolymers available for that purpose may well be typified by the group of products trademarked "VAGH" by Union Carbide Corp. of the United States.
Since the dispersibility of magnetic particles is improved by increasing the proportion of hydroxyl groups in the copolymers of the foregoing type, it has already been proposed to raise the OH/CH absorption ratio according to the infrared absorption spectrum to 0.7 or more (Japanese Patent Application Publication No. 3393/1981). A drawback to the proposal is that the copolymers with increased hydroxyl group contents are difficult to process on the surface because of their glass transition temperature as high as 70.degree. C. or above and hence the resulting magnetic layer after calendering can hardly be improved in surface quality. In an effort to eliminate the drawback, the present inventors previously proposed the use as a binder resin of a blend of a vinyl chloride-vinyl acetate-vinyl alcohol copolymer with a copolymer ingredient having a glass transition point of 65.degree. C. or below (Japanese Patent Application No. 94016/1981, etc.). Such a resinous mixture as a binder ingredient improves the coating surface properties and glossiness of the calendered surface without sacrifice of the magnetic powder dispersibility. However, the low glass transition temperature adversely affects the storability and running properties of the magnetic tapes under high-temperature, high-humidity conditions. In order to improve these properties of the tapes under the stringent conditions, the present inventors have also proposed the adoption as a binder resin of a copolymer having an OH/CH absorption ratio according to the infrared absorption spectrum of 0.2 to 0.7, the constituents of the copolymer being vinyl chloride, a vinyl alkylcarboxylate, a monomer copolymerizable with vinyl chloride, and a saponified vinyl alkylcarboxylate (Japanese Pat. App. No. 182254/1981).
In recent years, however, these resins have not always achieved satisfactory dispersion because more and more finely divided magnetic materials (having specific surface areas of at least about 30 m.sup.2 /g as determined by the BET method) have come into use for improvements in the S/N ratio. Emergence of a binder component capable of providing good dispersion under these conditions has therefore been eagerly awaited.